﻿Vol. 6 1 .] AKENIO FAWE AND MOEL LLYENANT. 623 



Lower Lianvirn fauna of the late Dr. Hicks, but is without 

 the Placoparia and other trilobites common in South Wales. The 

 Bifidus-Be&s are most unusually thin, but in their 20 or 30 feet 

 bear record of a very notable evolution of graptolite-life. Near 

 the base the commonest species is Didymograptus nanus, Lapw., 

 together with D. patulus, Hall, Gryptograptus tricornis, Carr., a 

 Dendrograptus, and Didymograptus bifidus, Hall, of the normal 

 small type. At the top, the vaguely-defined Diplograptus dentatus, 

 Brongn., which probably includes several species, is most abundant, 

 and with it are Didymograptus artus, E. & W.,D. acutidens, Lapw., 

 D. stabilis, E. & W., Glossograptus sp. and Gryptograptus tricornis, 

 Carr., and many specimens of Didymograptus bifidus, Hall, of a 

 type intermediate between D. bifidus and D. Murchisoni. At this 

 horizon also tails and other fragments of some Ogygia, like O.peltata, 

 Salt., or possibly 0. Buchii, Brongn., are not infrequent, and a Dionide 

 closely similar to those from Ty-obry has been observed. 



Like the Hirundo-Be&s below, the Bifidus-Beds are not well 

 exposed. Coming, however, immediately beneath the great volcanic 

 group, they have been traced more or less continuously all along 

 their outcrop. In the moorland between the Ffestiniog lioad and 

 Nant-yr-Olchfa, they appear upon their dip-slope and occupy a 

 considerable area. Where they just cross the burn the higher beds 

 are exceedingly fossiliferous, and the Drift of the whole area teems 

 with fossils. Under the slopes of Maen Grugog by Filltirgerig 

 they are again well seen, and yield a profitable return of the lower 

 graptolites. The Nant-Rhos-ddu section has been carefully de- 

 scribed by Miss Elles. 1 There the whole series is seen to be much 

 mixed with pyroclastic felspar, and includes also some bombs of 

 ash, a fact of considerable interest in determining the age and 

 focus of origin of the overlying volcanic group. 



The Volcanic Series. 



Upon the Bijidus -Shales come the earliest members of the great 

 volcanic group of Arenig, the Ashes and Porphyries of 

 Sedgwick. This is a series of very great thickness, but, owing to 

 the almost complete absence of fossils, it can only be subdivided 

 by complete field-mapping, according to the petrological character of 

 its component rocks. 



The Lower Ashes and Agglomerates [7]. 



The earliest of the eruptive products of Arenig is an inconstant 

 series of coarse, well-bedded and platy ashes. These, in the course of 

 some 30 feet or less, pass almost imperceptibly into the massive 

 agglomerate which forms the great bulk of the Lower Series. In 

 places, the Lower Platy Ashes seem to thin out, and especially in 

 the neighbourhood of certain of the contemporaneous intrusions to 

 be described hereafter (pp. 628-30), the Agglomerate comes to rest 

 directly upon the shales beneath. 



1 Geo!. Mag. 1904, pp. 205-207. 



